Alumina-forming austenitic (AFA) stainless steels are a new class of high-temperature (600-900° C.; 1112-1652° F.) structural alloy steels with a wide range of energy production, chemical/petrochemical, and process industry applications. Examples of such steels can be found in United States patents including U.S. Pat. No. 7,744,813, U.S. Pat. No. 7,754,144, and U.S. Pat. No. 7,754,305, the disclosures of which are incorporated fully by reference. These steels combine the relatively low cost, excellent formability, weldability, and good high-temperature creep strength (resistance to sagging over time) of state-of-the-art advanced austenitic stainless steels with fundamentally superior high-temperature oxidation (corrosion) resistance clue to their ability to form protective aluminum oxide (alumina, Al2O3) surface layers. Conventional high-temperature stainless steels rely on chromium-oxide (chromia, Cr2O3) surface layers for protection from high-temperature oxidation. However, compromised oxidation resistance of chromia in the presence of aggressive species such as water vapor, carbon, sulfur, and the like typically encountered in energy production and process environments necessitates a reduction in operating temperature to achieve component durability targets. This temperature reduction reduces process efficiency and increases environmental emissions.
Alumina grows at a rate 1 to 2 orders of magnitude lower than chromia and is also significantly more thermodynamically stable in oxygen, which results in its fundamentally superior high-temperature oxidation resistance. A further, key advantage of alumina over chromia is its greater stability in the presence of water vapor. Water vapor is encountered in most high-temperature industrial environments, ranging, for example, from gas turbines, combustion, and fossil-fired steam plants to solid oxide fuel cells. With both oxygen and water vapor present, volatile chromium oxy-hydroxide species can form and significantly reduce oxidation lifetime, necessitating significantly lower operating temperatures. This results in reduced process efficiency and increased emissions.
To date AFA alloy development has focused on wrought material forms (plate, sheet, foil, and tubes). However, many applications require complicated component shapes best achieved by casting (engine and turbine components). Casting can also result in lower cost tube production methods for chemical/petrochemical and power generation applications.